The present invention generally relates to a gas sensor for detecting a gas in an ambience, and particularly to a gas sensor having a metal-oxide semiconductor layer in which a characteristic thereof can be stable.
Conventionally, the following two types of gas sensors, in each of which a metal-oxide semiconductor is used as a gas sensitive material which can respond to a predetermined gas has been known. In a first type gas sensor, a heater layer is provided via an electrode layer and an insulating layer on a back of the metal-oxide semiconductor layer. In a second type gas sensor, electrodes, and heater coils are provided in a metal-oxide semiconductor block. The gas is absorbed by the metal-oxide semiconductor layer or block, so that a resistance value of the metal-oxide semiconductor layer which is heated by the heater layer or the heater coil is changed. That is, these types of gas sensors detect the gas based on the changing of the resistance value of the metal-oxide semiconductor.
The first type of gas sensor has, for example, a structure as shown in FIGS. 1A and 1B. FIG. 1A is a cross sectional view of the gas sensor, and FIG. 1B is a perspective view thereof.
Referring to FIGS. 1A and 1B, a heater layer 2 is formed on a substrate 1 having an adiabatic characteristic. An insulating layer 3 is formed on the heater layer 2 and electrode layers 41 and 42 are separated from each other and provided on the insulating layer 3. Then a metal-oxide semiconductor layer 51 is formed between the electrode layers 41 and 42 on the insulating layer 3 so that respective side portions of the metal-oxide layer 51 are in contact with the electrode layers 41 and 42, respectively. In a case where the substrate 1 is formed of a conductive material, another insulating layer needs to be provided between the substrate 1 and the heater layer 2. A wire 61 is bonded on an end portion of the heater layer 2 and a wire 62 is bonded on another end portion thereof. Power is supplied via the wires 61 and 62 from a power supply to the heater layer 2. That is, the wires 61 and 62 function as power supplying lines. A wire 71 is bonded on the electrode layer 41 and a wire 72 is bonded on the electrode layer 42. A detecting signal is generated between wires 71 and 72. The wires 71 and 72 function as signal output lines.
FIG. 2 shows another embodiment of the first type gas sensor. In FIG. 2, those parts which are the same as those shown in FIGS. 1A and 1B are given the same reference numbers.
In FIG. 2, the metal-oxide semiconductor layer 51 (a gas sensitive layer) is formed on the insulating layer 3 before the electrode layers 41 and 42 are formed thereon. That is, an edge portion of each of the electrode layers 41 and 42 is provided on a corresponding edge portion of the metal-oxide semiconductor layer 51.
The second type of gas sensor has, for example, a structure as shown in FIG. 3.
Referring to FIG. 3, four electrode pins 8a, 8b, 8c and 8d are provided on a base member 12 having a disc shape so that each of the electrode pins 8 projects from both sides of the base member 12. A gas sensitive material which is a sinter of the metal-oxide semiconductor is mounted on the base member 12. The gas sensitive material 52 has, for example, a 2 or 3 mm cubic shape. First and second heater coils 43 and 44 are laid in the gas sensitive material 52. The first heater coil 43 is connected between the electrode pins 8a and 8b, and the second heater coil 44 is connected between the electrode pins 8c and 8d. A detecting signal is output, for example, between the electrode pin 8b connected to the first heater 43 and the electrode pin 8d connected to the second heater 44.
In the second type of gas sensor described above, the gas sensitive material 52 formed of the metal-oxide semiconductor has a relatively large size, so that power is consumed by the first and second heaters 43 and 44. In addition, the heat capacity of the gas sensitive material 52 is large, so that the response characteristic is poor.
In the first type of gas sensor as shown in FIGS. 1A, 1B and 2, the gas sensitive material has a thin film shape, so that the power consumed by the heater layer 2 is small and the response characteristic is good. However, it is easy for a sensing characteristic, such as sensitivity with respect to the gas, to deteriorate with the passage of time, so that this first type of gas sensor has not been put into practical use yet.
The sensitivity with respect to the gas deteriorates when the size of each crystal particle of the metal-oxide semiconductor increases. The gas sensor is generally used under a condition of 300.degree.-450.degree. C., so that the size of each crystal particle of the metal-oxide semiconductor increases when the gas sensor is being used for a long time. Thus, when the size of each crystal particle of the metal-oxide semiconductor increases, an absorption area in which the gas should be absorbed decreases and the chemical activation of the metal-oxide semiconductor decreases. Therefor, the sensitivity with respect to the gas decreases.